The present invention relates to a combustor for a gas turbine combustion system and particularly relates to apparatus and methods for displacing a fuel nozzle and altering the gap in a venturi section of a gas turbine combustor during operation to vary performance and stability in the combustor and reduce NO.sub.x emissions.
One of the principal objectives in modern-day gas turbine manufacturing and gas turbine operation is to minimize emissions from nitrogen oxides (NO.sub.x) Many different concepts have been proposed and used for reducing such emissions, for example, by reducing flame temperature, residence time of the gases at peak temperatures, or by introducing water or steam into the flame. However, practical considerations preclude use of many of these proposals. For example, complexity of structure, higher operating costs and degradation of other performance parameters frequently occur when such proposals are adopted.
It has previously been found that a venturi configuration can be used to stabilize combustion flame. In such arrangements, reduced NO.sub.x emissions are achieved by lowering peak flame temperatures by burning a lean, uniform mixture of fuel and air. In the pre-mixed mode, fuel is supplied to both the primary and secondary nozzles (predominantly in the primary nozzle) and mixes in a pre-mixing chamber upstream of the venturi. The pre-mixed gases then pass through the venturi gap before igniting and combustion occurs downstream of the venturi gap.
It has been found that the venturi gap has an effect on the emissions in the pre-mixed mode. More particularly, it has been found that a smaller gap, when operating in the pre-mixed mode, provides reduced emissions. Recognizing this, however, means also to have recognized that the fuel nozzles, liners and various ancillary parts are conventionally rigidly secured within the combustor, with no purposeful or intended relative movement between such parts. Typically, relative movement of such parts is only incidental to operation of the combustor, i.e., a result only of thermal expansion. It has thus been found desirable to not only change the gap during operation in the pre-mixed mode but also to move the secondary fuel nozzle relative to the combustor end plate and the gap.
Therefore, in accordance with the present invention, there is provided a movable combustion system in the combustor of a gas turbine wherein the centerbody of the combustor upstream of the venturi is axially displaceable to alter the extent of the gap between the venturi and the centerbody, as well as axially displace the secondary fuel nozzle, all displacements being performed purposefully and intentionally during operation of the gas turbine. To accomplish this, the centerbody of the combustor is carried on an axially displaceable support element or pipe, which also carries the secondary fuel nozzle and supplies fuel thereto. The pipe is connected at its end passing through the combustor cover to an externally threaded centerbody support element, preferably a sleeve, for cooperation with a threaded member secured to and accessible from outside of the cover. The support element is keyed to the cover to prevent rotation of the centerbody during axial displacement thereof. Consequently, by rotating the internally threaded member outside of the cover, the centerbody support element carrying the secondary fuel nozzles, as well as ancillary structure including the inner liner, swirler blades and other structure, are axially displaced relative to the cover, venturi and primary fuel nozzle. Thus, the downstream end of the centerbody is adjusted axially relative to the venturi whereby the gap between the venturi and the centerbody end as well as the location of the secondary fuel nozzles may be adjusted during operation.
In a preferred embodiment according to the present invention, there is provided a combustor assembly for a gas turbine comprising a combustor body having an outer liner, a centerbody carrying an inner liner and a cover, and arranged about an axis, means carried by the assembly for supplying fuel within the combustor body, means for supplying air within the combustor body, means defining a venturi and means including a portion of the centerbody defining a gap with the venturi. Means are also provided external to the combustor body and connected to the centerbody for moving the centerbody in an axial direction for changing the size of the venturi gap.
In a further preferred embodiment according to the present invention, there is provided a combustion assembly for a gas turbine comprising a combustion body having an outer liner, a centerbody carrying an inner liner and a cover, means for supplying fuel within the combustor body including a fuel nozzle and means external to the combustor body and connected to the fuel nozzle for moving the fuel nozzle in an axial direction for changing the axial location of the fuel nozzle relative to the combustor body.
In a further preferred embodiment according to the present invention, there is provided a method of operating a combustor for a gas turbine wherein the combustor has a fuel/air pre-mixing chamber, a combustor chamber downstream from the pre-mixing chamber and a venturi, comprising the steps of flowing the fuel/air mixture into the combustion chamber through a gap formed by a fixed surface of the venturi and a movable surface and altering the size of the gap by displacing the movable surface relative to the fixed surface.
In a further preferred embodiment according to the present invention, there is provided a method of operating a combustor for a gas turbine wherein the combustor has a fixed primary fuel nozzle adjacent a forward end of the combustor and a movable secondary fuel nozzle axially downstream from the primary fuel nozzle comprising the step of axially displacing the secondary fuel nozzle relative to the primary fuel nozzle during operation of said gas turbine.
Accordingly, it is a primary object of the present invention to provide novel and improved apparatus and methods for displacing the centerbody of a combustor thereby to displace the secondary fuel nozzle relative to the cover and alter the gap in the venturi as desired in a dry, low NO.sub.x turbine and during operation.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.